In the production of oil from subterranean formations, it is usually possible to recover only a small fraction of the total oil present in the formation by so-called primary recovery methods which utilize only the natural forces present in the reservoir. To recover oil beyond that produced by primary methods, a variety of supplemental production techniques have been employed. In these supplemental techniques, commonly referred to as secondary or tertiary recovery operations, a fluid is introduced into the oil-bearing formation in order to displace oil to a production system comprising one or more production wells. The displacing or "drive" fluid may be an aqueous liquid such as brine or fresh water, a gas such as carbon dioxide, steam or dense-phase carbon dioxide, an oil-miscible liquid such as butane, or an oil and water-miscible liquid such as an alcohol. Often, the most cost-effective and desirable secondary recovery methods involve the injection of steam. In practice, a number of injection and production wells will be used in a given field arranged in conventional patterns such as a line drive, a five spot or inverted five spot, or a seven spot or inverted seven spot.
In the use of the various flooding techniques, it has become a common expedient to add various polymeric thickening agents to the drive fluid to increase its viscosity to a point where it approaches that of the oil which is desired to be displaced, thus improving the displacement of oil from the formation. The polymers used for this purpose are often said to be used for "mobility" control.
Another problem encountered is that certain injected drive fluids may be much lighter than the reservoir fluids and thus separate by gravity, rising toward the top of the flowing region and resulting in the bypassing of the lower regions. This phenomena is known as gravity override.
Also encountered in the use of the various flooding techniques is a situation caused by the fact that different regions or strata often have different permeabilities. When this situation is encountered, the drive fluid may preferentially enter regions of higher permeability due to their lower resistance to flow rather than the regions of low permeability where significant volumes of oil often reside.
It therefore is often desirable to plug the regions of high permeability, or "thief" zones, either partly or entirely, so as to divert the drive fluid into regions of lower permeability. The mechanical isolation of these thief zones has been tried but vertical communication among reservoir strata often renders this method ineffective. Physical plugging of the high permeability regions by cements and solid slurries has also been tried with varying degress of success; however, these techniques have the drawback that still-productive sites may be permanently closed.
As a result of these earlier efforts, the desirability of designing a system capable of sealing off the most permeable layers so that the drive fluid would be diverted to the underswept, "tighter" regions of the reservoir, became evident. This led to the use of oil/water emulsions, as well as gels and polymers for controlling the permeability of the formations. This process is frequently referred to as "flood conformance" or "profile control", a reference to the control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed include oil/water emulsions, gels, e.g., lignosulfate gels and polymeric gels, with polymeric gels being the most extensively applied in recent years.
Among the polymers so far examined for improving flood conformance are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates. A major part of this work has been conducted with the polyacrylamides, both in their normal, non-crosslinked form, as well as in the form of metal complexes, as described, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680. In either form, the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection and sensitivity to reservoir brines, low pH and high temperature. To overcome these problems and to achieve deeper polymer penetration into the reservoir, dilute solutions of these polymers have sometimes been injected first and then complexed in situ.
Another group of polymeric thickeners which has received considerable attention for use in improving flooding are polysaccharides, particularly those produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose a process for mobility control by the use of polysaccharides.
U.S. Pat. Nos. 3,741,307, 4,009,755 and 4,069,869 disclose the use of polysaccharides in the control of reservoir permeability. U.S. Pat. No. 4,413,680 describes the use of crosslinked polysaccharides for selective permeability control in oil reservoirs.
U.S. Pat. No. 3,908,760 describes a polymer waterflooding process in which a gelled, water-soluble Xanthomonas polysaccharide is injected into a stratified reservoir to form a slug, band or front of gel extending vertically across both high permeability and low permeability strata. This patent also suggests the use of complexed polysaccharides to block natural or man-made fractures in formations. The use of polyvalent metal ions for cross-linking polysaccharides is also disclosed in U.S. Pat. No. 3,810,882.
Another type of polysaccharide which has been experimented with in the area of profile control is the non-xanthan, heteropolysaccharide S-130. S-130 belongs to the group of welan gums and is produced by fermentation with a microorganism of the genus Alcaligenes. Another welan gum heteropolysaccharide, known as S-194, is also produced by fermentation with a microorganism of the genus Alcaligenes. A notable characteristic of the heteropolysaccharide S-130 is that it develops a high viscosity in saline waters. This is particularly so in brines which contain divalent cations such as Ca.sup.2+ and Mg.sup.2+ or monovalent cations such as Na.sup.+ and K.sup.+. U.S. Pat. No. 4,658,898 discloses the use of welan gum S-130 in saline waters. Crosslinking with trivalent cations, such as chromium, aluminum, zirconium and iron is also disclosed. Additionally, crosslinking with organic compounds containing at least two positively charged nitrogen atoms is disclosed in U.S. Pat. No. 4,658,898; while Ser. No. 283,399, filed on Dec. 12, 1988, discloses welan gums crosslinked with phenolic resins or mixtures of phenols and aldehydes.
The use of various block copolymers for mobility control in waterflooding operations is described in U.S. Pat. Nos. 4,110,232, 4,120,801 and 4,222,881. Chung et al., U.S. Pat. No. 4,653,585, disclose the use of block copolymers, which may be crosslinked with polyvalent metal ions, for use as permeability control agents in enhanced oil recovery applications.
While a number of the different compositions discussed have been proposed for permeability control, some of these compositions may be unsuitable for use as permeability control agents under certain circumstances. For example, the polymers of Chung et al., may not be effectively crosslinked with polyvalent metal ions under all conditions encountered in the enhanced oil recovery applications, e.g., in acidic conditions commonly found in carbon dioxide (CO.sub.2) flooding operations. Polyacrylamides display instability in the presence of high brine concentration at temperatures over 70.degree. C. Xanthan gums are very brine tolerant but display thermal instability, even at temperatures below 60.degree. C. Still, other polymers are unsuitable as permeability control agents when used in conjunction with steam flooding operations. This is due to the fact that they lose their structural integrity (i.e., they undergo "syneresis") at the high temperatures generated during such operations.
Syneresis is the contraction or shrinking of a gel so that liquid is exuded at the gel surface. For example, a gel said to exhibit 20% syneresis would take up 80% of its original volume, with the remaining 20% being expelled water. Although the exact mechanism responsible for the syneresis of such gel-forming compositions is not fully understood, it is believed to result from the over-crosslinking of the polymeric material with time. While it is not yet known what an acceptable level of syneresis might be for profile control gels, it is believed that to minimize syneresis would enhance the effectiveness of such gels.
In view, therefore, of the severe conditions which include both high brine concentrations and elevated temperatures, there is a need for brine tolerant, thermally- stable materials suitable for high temperature wells and steam flooded wells. This has led to the development of the so-called hostile environment polymers, such as those marketed by the Phillips Petroleum Company of Bartlesville, OK and the Hoechst Celanese Corporation of Somerville, N.J.
It has been known to use polyvinyl alcohol (PVA) metal-crosslinked gels, as described in U.S. Pat. No. 3,762,476 to Gall, in order to correct subterranean formation permeability. See also U.S. Pat. No. 4,039,029 to Gall and U.S. Pat. No. 4,018,286 to Gall, et al. Unfortunately, polyvinyl alcohol has a drawback in that it very poor cold water solubility, and has to be dispersed in cold water as a suspension followed by heating to a temperature of, for example, 90.degree.-95.degree. C. for dissolution. Furthermore, polyvinyl alcohol also has the limitation that the choice of possible crosslinking metal components is limited, titanium (IV) probably being the only practical choice. Other commonly used metals such as aluminum, chromium, and zirconium are not effective to crosslink polyvinyl alcohol. Furthermore, polyvinyl alcohol gels crosslinked with titanium are not brine stable at high temperatures. In particular, syneresis can occur.
Polyvinyl alcohol can also be crosslinked covalently with polyaldehydes, as described in the GB Patent Application No. 2,145,420 A. of Marocco et al. The gelation described in the Marrocco et al. disclosure, however, requires an acidic environment, creating a need for controlling the pH of the reservoir to facilitate the process. Needless to say, control of the reservoir environment at a particular pH can be a very difficult task. Regardless of the alternative crosslinking process described by Marrocco et al., the polyvinyl alcohol solution preparation problems remain and the gel stability in strong brine concentrations is uncertain.
Ser No. 092,274, filed on Sept. 4, 1987, the inventors of which are also the inventors of the present invention, discloses crosslinked polymers obtained by crosslinking polyvinyl alcohol or a derivative thereof with a crosslinking agent which is a mixture of a phenolic component and an aldehyde or a mixture of a naphtholic component and an aldehyde. While such crosslinked polymers are stable even at the high underground formation temperatures encountered during steam flooding oil recovery operations, the polyvinyl alcohol solution preparation problems persist for many of the compositions disclosed. Ser. No. 092,274 is hereby incorporated by reference in its entirety.
Polyvinyl alcohol is not a very water soluble polymer. This is because its compact structure and many hydrogen bonds lead to high crystal energy [See, T. H. Kwei in "Macromolecules, An Introduction to Polymer Science", (F. A. Bovery and F. H. Winslow, ed.) Academic Press, N.Y., 1979, p. 273]. Disrupting the crystal packing of polyvinyl alcohol, such as by copolymerizing with a suitable monomer can greatly improve the resultant copolymer's solubility in water.
U.S. Pat. No. 4,678,032, the inventor of which is a co-inventor of the present invention, teaches multivalent transition metal crosslinked polyvinyl alcohol copolymeric gels useful for profile control under severe reservoir conditions. The copolymers taught are selected from the groups consisting of poly (vinylalcohol-co-vinyl-carboxyl and poly (vinylalcohol-co-vinylether). U.S. Pat. No. 4,678,032 is hereby incorporated by reference in its entirety.
Despite these developments, a need still exists for permeability control agents which are easy to prepare and are compatible with the harsh conditions encountered in steam flooding enhanced oil recovery operations.
It is, therefore, an object of the present invention to provide a polymer gel-forming composition for injection in a subterranean reservoir which has good thermal stability at high brine concentration.
It is another object of the present invention to provide a polymer gel which can be used effectively as a permeability control agent under the extreme temperature conditions encountered in the steam flooding of underground formations.
It is a further object of the present invention to provide an aqueous crosslinked polyvinyl alcohol copolymer gel which is easily prepared.
Other objects and the several advantages of the present invention will become apparent to those skilled in the art upon a reading of the specification and the claims appended thereto.